The invention relates to a positioning controller for performing positioning operation by driving load through use of a motor, and more particularly, to a technique for damping low-frequency vibration in a load or in a bed on which the load is to be disposed.
In pursuit of a demand for weight reduction and miniaturization of recent industrial machinery, the rigidity of a shaft used for connecting a motor with load and that of a bed on which a drive mechanism is to be disposed are becoming lower, and the natural frequency of a load system and that of the bed tend to decrease. In tandem with these tendencies, the performance of a servo control system and a response in the drive system also tend to be enhanced. In relation to the servo control system remaining under these circumstances, a problem of the load or the bed becoming susceptible to vibration is starting to become noticeable.
In order to dampen such vibration, a servo controller, such as that shown in FIG. 3, described in xe2x80x9cMaterials for Workshop in The Institute of Electrical Engineers of Japanxe2x80x9d IIC-96-17, pp. 75 to 84, has already been put forth. In the drawing, reference numeral 31 designates an S-shaped position command generator; 32 designates a metastable pre-filter; 33 designates a semi-closed feedback position control system; 33a designates a transfer function from a position command of a feedback control system to the position of a motor; and 33b designates a transfer function from the position of the motor to the position of load. A natural vibration angular frequency of load in the feedback control system is xcfx89a, which shows a case where a speed pattern xcex8*M of an S-shaped position command assumes a trapezoidal shape as shown in FIG. 5, wherein an acceleration time and a deceleration time are equal to each other. Here, tA denotes an acceleration/deceleration time; and (tDxe2x88x92tA) denotes a constant speed time.
When the servo controller has such a control system, a vibration pole xcfx89a of the feedback position control system 33 is canceled by a zero of a transfer function of the metastable pre-filter 32, wherein the transfer function is defined as                                           F            1                    ⁡                      (            S            )                          =                                                            S                2                            +                              ω                a                2                                                                    S                2                            +                              ω                                  a                  ⁢                                      xe2x80x83                                    ⁢                  n                                2                                              ·                                                    ω                                  a                  ⁢                                      xe2x80x83                                    ⁢                  n                                2                                            ω                                  a                  ⁢                                      xe2x80x83                                                  2                                      .                                              (        1        )            
Dampening control is accomplished by setting xcfx89an so as to satisfy any term of Equation (2).
1xe2x88x92cos xcfx89antA=0, 1xe2x88x92cos xcfx89antD=0xe2x80x83xe2x80x83(2)
The related art presents the following problems.
(1) When the acceleration/deceleration time and the constant speed time, both pertaining to the target position command xcex8*M, are short, a position command xcex8**M of the feedback system becomes steep (see FIG. 6, and detailed descriptions will be provided later).
(2) Equation (2) must be satisfied strictly. If Equation (2) is not satisfied, continuous vibrations develop. For instance, if the acceleration/deceleration time of the target position command xcex8*M has been nominally changed by 5%, the position command xcex8**M of the feedback system will continuously vibrate as shown in FIG. 7, which will be described later.
(3) If an attempt is made to implement the related-art metastable prefilter through use of computation means such as a CPU or the like, a critical stability pole of the metastable prefilter becomes unstable, for reasons of an error in a discrete approximations, which in turn makes the metastable prefilter unstable. For instance, even if a sampling time is nominally 100 xcexcs, the position command xcex8**M of the feedback system becomes unstable as shown in FIG. 8 (i.e., the position command becomes divergent while vibrating).
When consideration is given to the problems of the related-art metastable prefilter, the true nature of all the problems resolves itself to a 0 attenuation term of the denominator of the filter. Simple application of a filter having an attenuation term to the metastable prefilter is usually conceivable as a solution. However, this solution ends in an increase in a delay time of the filter, thereby prolonging a positioning time. In the related-art, the attenuation term is considered to be set to zero for eliminating such a delay time. Therefore, following the conventional line of thought cannot produce the configuration of a prefilter xe2x80x9chaving high stability and involving a short positioning time.xe2x80x9d
In order to solve such a basic problem, the invention aims at providing a vibration-damping positioning controller which can fix a position command of a feedback control system to a target position by setting an output from a prefilter to zero when the position command of the feedback control system coincides with a target command, thereby enabling stable positioning operation within a short period of time.
A vibration-damping positioning controller according to an invention of claim 1 produces a position command when given a differential value of a target position command that changes in a predetermined pattern, and causes load to follow the position command by inputting the position command to the feedback control system, wherein the controller comprises: an integrator for integrating a differential value of the target position command; a subtracter which subtracts the position command from an output of the intergrator, to thereby output a position command error; a stable prefilter which receives a differential value of the target position command and eliminates a frequency component identical with a vibration pole xcfx890 of the feedback control system; command fixing means which receives an output from the stable prefilter and the position command error, judges that the command is started when a differential value of the target position command has changed from zero to a non-zero value, judges that the command is ended when the differential value of the target position command has changed from the non-zero value to zero, outputs zero from the time the position command error has assumed zero after the command is ended until the next command is started, and in other cases produces an output of the stable prefilter in an unmodified form; and a second integrator which integrates an output from the command fixing means, to thereby out put the position command.
A vibration-damping positioning controller according to an invention of claim 2 produces a position command when given a target position command that changes in a predetermined pattern, and causes load to follow the position command by inputting the position command to the feedback control system, wherein the controller comprises: a differentiator for differentiating the target position command; a subtracter which subtracts the position command from the target position command, to thereby output a position command error; a stable prefilter which receives a differential value of the target position command serving as an output from the differentiator and eliminates a frequency component identical with a vibration pole xcfx890 of the feedback control system; command fixing means which receives an output from the stable prefilter and the position command error, judges that the command is started when a differential value of the target position command has changed from zero to a non-zero value, judges that the command is ended when the differential value of the target position command has changed from a non-zero value to zero, outputs zero from the time the position command error has assumed zero after the command is ended until the next command is started, and in other cases produces an output of the stable prefilter in an unmodified form; and a second integrator which integrates an output from the command fixing means, to there by output the position command.
By means of the foregoing configuration, even when low-frequency vibrations are present in load or a bed on which the load is to be placed, stable positioning can be performed within a short period of time.
As mentioned above, according to the invention, a target command signal is caused to pass through a stable prefilter. Hence, a natural vibration frequency component of machinery included in the target command signal is omitted. Further, the command signal that has passed through the stable prefilter is stable and smooth. Moreover, the command signal that has passed through the stable prefilter is caused to pass through the command fixing section. Hence, the final command signal that has been applied to the feedback control system reaches the target position immediately. Accordingly, there is yielded an advantage of the ability to obtain a stable positioning command whose command fixing time is short without inducing mechanical vibrations.